This application is based on application No. 99-20982 filed in the Korean Industrial Property Office on Jun. 7, 1999, the content of which is incorporated herein by reference.
(a) Field of the Invention
The present invention relates to an electrolyte for a rechargeable lithium battery and a rechargeable lithium battery using the same, and more particularly, to an electrolyte for a rechargeable lithium battery exhibiting high capacity.
(b) Description of the Related Art
In recent years, the development of miniaturized portable electronics provokes needs for a rechargeable battery having a high capacity as well as a light weight. From the viewpoint of the capacity improvement per unit weight, the rechargeable lithium battery is preferably adopted because lithium has a high standard potential as well as a low electrochemical equivalent weight.
Metallic lithium has been used for the negative active material in rechargeable lithium batteries. However, during charging, lithium is electroplated onto the metallic lithium electrode, it deposits to form a dendric layer which induces a short circuit between the negative and positive active material. The problem of lithium reactivity toward the electrolyte is addressed by replacing lithium metal with carbon-based materials.
For the positive active material in the rechargeable lithium battery, transition metal oxides are used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2(0 less than X less than 1) or LiMnO2. Manganese-based material such as LiMn2O4 or LiMnO2, is easiest to prepare, is less expensive the other materials and has environmentally friendly characteristics, but it has low capacities. LiCoO2 is widely used as it has a good electrical conductivity and high battery voltage, but it is expensive. LiNiO2 is relatively inexpensive and has a high charge capacity, but it is difficult to produce and has safety problems.
The electrochemical performances of rechargeable lithium batteries depend on the reaction between the positive active material and an electrolyte or between the negative active material and an electrolyte such that the choice of electrolyte, particularly organic solvent is critical to battery performance. There have been attempted to use various organic solvents for electrolytes. It is understood that when an one organic solvent is used alone, rechargeable lithium batteries exhibiting excellent electrochemical performances cannot be obtained, so mixed organic solvents are used.
In addition, the cycle life and capacity of the lithium ion battery greatly depend on the surface reactivity between the electrolyte and the negative active material. Accordingly, the reactivity between the electrolyte and the negative active material rather than the positive active material should be considered when developing the electrolyte composition.
It has been proposed in U.S. Pat. No. 5,437,945 (Sony) to use two components for electrolytes. The electrolyte includes propylene carbonate and methyl ethyl carbonate to improve low-temperature characteristics. Propylene carbonate exhibits good low-temperature characteristics, but has high reactivity with graphite negative active material, and methyl ethyl carbonate has low reactivity with graphite negative active material, but exhibits poor low-temperature characteristics. However, it is insufficient to obtain rechargeable lithium batteries with the desired performance.
To improve low-temperature characteristics, a three-component mixed organic solvent including a solvent having a low freezing point is also used for electrolytes (U.S. Pat. No. 5,474,862 assigned to Matsushita). However, it is still insufficient to provide a rechargeable lithium battery exhibiting good electrochemical properties.
It is an object of the present invention to provide an electrolyte for a rechargeable lithium battery exhibiting good low-temperature characteristics and high capacity.
It is another object to provide a rechargeable lithium battery using the same.
These and other objects may be achieved by an electrolyte for a rechargeable lithium battery including a cyclic carbonate, at least two linear carbonates selected from diethyl carbonate, methyl propyl carbonate, ethyl methyl carbonate or dimethyl carbonate, propyl acetate, and a lithium salt as a solute.
The present invention provides a rechargeable lithium battery including a positive electrode including a transition metal oxide, a negative electrode including carbonaceous negative active materials from or into which lithium ions are deintercalated or intercalated; and the electrolyte.